Does anybody know about water heaters?

It sounds as if your sink is right next to your boiler while your shower is some distance away. I’d consider an instant shower that gives hot water on demand.

The sink in the kitchen? Or, sink in the same bathroom?

Do you have one hot water heater for the entire house? If you have two, it sounds like the one for the sink is set way too hot, and the one for the shower way to cool.

I agree with @flutherother…the location, and possibly the size, of your hot water tank plays a big part. The water heated in the tank has to get to the faucet being used. It has to travel through pipes for this and the pipes are generally not heated. So the hot water transfers heat to the metal of the pipe, cooling the water. The farther it has to flow, the cooler it will get. If the hot water tank is far away from your shower, it will take longer to get up to temperature. And if the tank isn’t big enough, it may run out of hot water before the shower ever does get to temp. Conversely, if a sink is significantly closer to the hot water tank, it will see the full temp water sooner than the shower would, so you could see really hot water there and none in the shower.

You may have more than one heater. Or you may have a “on demand” heater for the sink. The shower sounds like that heater has one bad element and may be about to fail completely.

I have 1 water heater. It’s probably substandard, considering the age of my house. But it’s probably about the same distance from my shower, to my kitchen sink.

Just a note about water temp.
A plumber fixing my bathtub asked me how hot I want it set too?
I had not realized that he could do this ?
He simply dialed or turned a small thingy on the pipe that hooked to the tub.
It work from then on we had more heat as before it was always luke warm.
Check it out maybe not all have this ?

Distance from the water heater can be deceptive. The water most most likely branches off from a main pipe, and the branch point might be closer to the sink than the shower, even though if you are just walking through the house it’s the same distance.

Still, the distance from the water heater should effect how long it takes for the hot water to start running through the faucet, not how long the hot water lasts.

A shower will have a shorter amount of hot water minutes than a sink, because the shower has more water coming through it.

Your situation is a little screwy.

When you take a shower is it right after running the washing machine or dishwasher? Or, are you running those appliances at the same time as your shower?

I wonder if it is something with the fixture in your shower? Maybe it’s not completely opening the hot water side? If it’s a single handle, the apparatus inside can slip so to speak, and that will cause it to malfunction.

I don’t have a washer machine, or dishwasher. I try to just turn on the hot water side, right before I get in the shower. Then, I just have to hurry….